Abstract: The present invention relates to a process for the synthesis of overbased additives obtained by carbonation under constant pressure of carbon dioxide, the process consisting of carbonating a reaction mixture comprising a derivative of an alkaline-earth metal and at least one surfactant in a diluent oil and a hydrocarbon solvent in the presence of an oxygeneous accelerator and a nitrogeneous accelerator, while maintaining a constant pressure of carbon dioxide above the reaction mixture, the carbon dioxide is thus progressively introduced as it is consumed, thereby allowing its concentration in the liquid medium not to become too high which could thereafter provoke the formation of insoluble over-carbonated products, the reaction to be self-controlled and to obtain excellent yields of overbased products having very high alkaline values and alkaline-earth contents, while however remaining fluid and stable.